Ben Van de Wiele

Electric field driven magnetic domain wall motion in ferromagnetic-ferroelectric heterostructures

We investigate magnetic domain wall (MDW) dynamics induced by applied electric fields in ferromagnetic-ferroelectric thin-film heterostructures. In contrast to conventional driving mechanisms where MDW motion is induced directly by magnetic fields or electric currents, MDW motion arises here as a result of strong pinning of MDWs onto ferroelectric domain walls (FDWs) via local strain coupling. By performing extensive micromagnetic simulations, we find several dynamical regimes, including instabilities such as spin wave emission and complex transformations of the MDW structure. In all cases, the time-averaged MDW velocity equals that of the FDW, indicating the absence of Walker breakdown.

Fast numerical three-dimensional scheme for the simulation of hysteresis in ferromagnetic grains

The magnetic behavior of ferromagnetic materials like iron is important for the improvement of performances of electromagnetic devices. This article deals with a three-dimensional model that computes the magnetic behavior of an iron grain under a varying applied field starting from its microscopic material parameters. The model can be used as a tool to determine the quantitative relations between the different microscopic parameters and their influence on the magnetic properties of the grain. The magnetization dynamics are computed for successive quasistatic applied fields. In each space point of the grain, the time variation of the magnetic dipole M is described by the Landau-Lifshitz equation. To integrate this equation, two time stepping schemes are proposed: a forward semianalytical and a predictor-corrector time stepping scheme. The two methods obey the two intrinsic properties of the Landau-Lifshitz equation: (1) preservation of the amplitude of the magnetic dipoles and (2) the decrease of the total...

Tunable short-wavelength spin wave excitation from pinned magnetic domain walls

Miniaturization of magnonic devices for wave-like computing requires emission of short-wavelength spin waves, a key feature that cannot be achieved with microwave antennas. In this paper, we propose a tunable source of short-wavelength spin waves based on highly localized and strongly pinned magnetic domain walls in ferroelectric-ferromagnetic bilayers. When driven into oscillation by a microwave spin-polarized current, the magnetic domain walls emit spin waves with the same frequency as the excitation current. The amplitude of the emitted spin waves and the range of attainable excitation frequencies depend on the availability of domain wall resonance modes. In this respect, pinned domain walls in magnetic nanowires are particularly attractive. In this geometry, spin wave confinement perpendicular to the nanowire axis produces a multitude of domain wall resonances enabling efficient spin wave emission at frequencies up to 100 GHz and wavelengths down to 20 nm. At high frequency, the emission of spin waves in magnetic nanowires becomes monochromatic. Moreover, pinning of magnetic domain wall oscillators onto the same ferroelectric domain boundary in parallel nanowires guarantees good coherency between spin wave sources, which opens perspectives towards the realization of Mach-Zehnder type logic devices and sensors.

Effect of disorder on transverse domain wall dynamics in magnetic nanostrips

We study the effect of disorder on the dynamics of a transverse domain wall in ferromagnetic nanostrips, driven either by magnetic fields or spin-polarized currents, by performing a large ensemble of graphics processing unit-accelerated micromagnetic simulations. Disorder is modeled by including small, randomly distributed nonmagnetic voids in the system. Studying the domain wall velocity as a function of the applied field and current density reveals fundamental differences in the domain wall dynamics induced by these two modes of driving: For the field-driven case, we identify two different domain wall pinning mechanisms, operating below and above the Walker breakdown, respectively, whereas for the current-driven case pinning is absent above the Walker breakdown. Increasing the disorder strength induces a larger Walker breakdown field and current, and leads to decreased and increased domain wall velocities at the breakdown field and current, respectively. Furthermore, for adiabatic spin-transfer torque, the intrinsic pinning mechanism is found to be suppressed by disorder. We explain these findings within the one-dimensional model in terms of an effective damping parameter alpha* increasing with the disorder strength.

Energy considerations in a micromagnetic hysteresis model and the Preisach model

Finding the relations between the microstructural material parameters and the macroscopic hysteresis behavior is indispensable in the design of ferromagnetic materials with minimal (hysteresis) losses. Micromagnetic hysteresis simulations enable a rigorous and structured investigation of these relations since in the numerical model each material parameter can be altered independently. This paper describes a procedure to extract the Preisach distribution function, quantifying the macroscopic hysteresis properties, from micromagnetic simulations incorporating the materials’ microstructure. Furthermore, the instantaneously added, stored and dissipated energy while running through the hysteresis loop as described in the macroscopic Preisach model and in the micromagnetic hysteresis model are compared, evidencing a very good agreement. Moreover, using the micromagnetic model, the energy rearrangements between the different micromagnetic interaction terms is studied at each time point of the hysteresis loop. It...

Influence of material defects on current-driven vortex domain wall mobility

Many future concepts for spintronic devices are based on the current-driven motion of magnetic domain walls through nanowires. Consequently a thorough understanding of the domain wall mobility is required. However, the magnitude of the nonadiabatic component of the spin-transfer torque driving the domain wall is still debated today as various experimental methods give rise to a large range of values for the degree of nonadiabaticity. Strikingly, experiments based on vortex domain wall motion in magnetic nanowires consistently result in lower values compared to other methods. Based on the micromagnetic simulations presented in this contribution we can attribute this discrepancy to the influence of distributed disorder which vastly affects the vortex domain wall mobility, but is most often not taken into account in the models adopted to extract the degree of nonadiabaticity.

Memory properties in a Landau-Lifshitz hysteresis model for thin ferromagnetic sheets

The paper deals with a two-dimensional numerical model for the evaluation of the electromagnetic hysteretic behavior of thin magnetic sheets when applying a unidirectional magnetic field. The time variation of the magnetization vector m in each space point obeys the Landau-Lifshitz equation. The effective field is the result of several contributions: the applied field, the magnetostatic field, the anisotropy field, and the exchange field. Microstructural features, such as grain size and crystallographic texture, are introduced in the micromagnetic model by dividing the geometry in subregions, each with its own magnetic preferable directions. In the article, numerical experiments are presented aiming at low-frequency applications. The presented micromagnetic model is used to study magnetic memory material properties.

How finite sample dimensions affect the reversal process of magnetic dot arrays

We investigate the magnetization reversal of a magnetic dot array by means of magneto-optical Kerr effect and magnetic force microscopy measurements as well as micromagnetic simulations. We find that the finite dimensions of the dot array introduce a global configurational anisotropy that promotes state transitions first in dots near the sample boundaries. From there, the reversal process expands towards the sample body by means of collective magnetization processes originating in the magnetostatic coupling between the dots. These processes are characterized by transition avalanches and the formation of magnetization chains. These findings are important in the development of applications that rely on a robust control of dot magnetization states in dot arrays.

Space mapping methodology for defect recognition in eddy current testing – type NDT

Purpose – The purpose of this paper is to solve an inverse problem of structure recognition arising in eddy current testing (ECT) – type NDT. For this purpose, the space mapping (SM) technique with an extraction based on the Gauss‐Newton algorithm with Tikhonov regularization is applied.Design/methodology/approach – The aim is to have a computationally fast recognition procedure of defects since the monitoring results in a large amount of data points that need to be analyzed by 3D eddy current model. According to the SM optimization, the finite element method (FEM) is used as a fine model, while the model based on an integral method such as the volume integral method (VIM) serves as a coarse model. This approach, being an example of a two‐level optimization method, allows shifting the optimization load from a time consuming and accurate model to the less precise but faster coarse surrogate.Findings – The application of this method enables shortening of the evaluation time that is required to provide the p...

Creep turns linear in narrow ferromagnetic nanostrips

The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.